1. Field of the Invention
The present invention relates to a method for forming a beam of an array antenna and an apparatus therefor, and, in particular, to a method for forming a beam of an array antenna with the use of a null forming method, and an apparatus therefor.
2. Description of the Related Art
In wide-band radio communication, decrease in a service radius along with increase in transmission band width due to a limitation to transmission power may be a problem. In particular, in a cellular system, a limitation is applied to transmission power from a mobile station on the order of hundreds of milliwatts utmost. Therefore, uplink/downlink asymmetrical communication is supported by a wide-band radio system on a third generation (3G) or a higher generation. By setting an uplink transmission band width toward a base station from a mobile station lower than downlink transmission band width from the base stain to the mobile station, peak transmission power in the mobile station may be reduced. However, since a transmission rate, i.e., a transmission band width increases tens through hundreds of times, it is difficult to keep a cell radius (service radius) as it is.
For example, according to ‘Future Outlook of Mobile Communication’, written by Fumiyuki Adachi, Spectrum Spread Society Conference Paper, October 2000, of Institute of Electronics, Information and Communication Engineers, upon comparison between 3G (transmission rate of 384 kbps in fc=2 GHz) and a public service in a next-generation mobile communication system for which study has been started recently, i.e., 4G (transmission rate of 100 Mbps in fc=5 GHz), power in approximately 2800 [=(2.5)2.6×260] times will be required, according to a law of ‘fc2.6×Rate’. In other words, the transmission power increases by a degree equivalent to 35 dB.
Such a situation cannot be permitted in consideration of peak transmission power of a 3G terminal on the order of 0.2 W, since 560 W is required instead. In other words, assuming 50 Mbps, which is a half, to be provided for the uplink in the asymmetrical link system, improvement more than 30 dB is required. Further, by applying a 3.5-th power law to a propagation loss, a cell radius is reduced on the order of ⅛ through 1/10. Then, assuming that the cell radius in 3G is 5 km, a cell radius on the order of 500 m through 600 m is presumed. Since reduction in the cell radius increases the required number of base stations per unit area to the second power, improvement of power efficiency is indispensably demanded, i.e., it is necessary to increase in the cell radius on a condition of fixed peak transmission power, by increasing a gain of a directive antenna such as an adaptive array antenna, in order to achieve a seamless service in a wide-band radio system.
For the adaptive array antenna (AAA), there are two approaching methods, i.e., a ‘beam steering method’ of directing a directive beam to a communication target and increasing a signal (S) factor in a ratio [S/(I+N)]of the Desired signal (S), an interference signal (I) and a noise (N); and a ‘null steering (null forming) method’ of suppressing interference signals from other cells or other users, and thus suppressing the interference signal (I) factor in the ratio [S/(I+N)]. The present invention particularly relates to the latter method.
FIG. 1 shows a block diagram of an apparatus employing the null forming method according to the related art. With the use of N antennas, 101 through 10n, a weight vector WT for transmission beam forming given to multipliers 121 through 12n is expressed as |wT-1, wT-2, . . . , wT-N|, and a weight vector WR for reception beam forming given to multipliers 141 through 14n is expressed by |wR-1, wR-2, . . . , wR-N|. An arrival direction estimation part 16 estimates a path arrival direction (DoA: Direction of Arrival), from a mobile station acting as interference. A convergence algorithm part 18 operates a convergence algorithm such as that of a steepest descent method (LMS: Least Mean Squire) based on the path arrival direction DoA, and carries out reception null forming. Thereby, the weight vector WT for transmission beam forming and the weight vector WR for reception beam forming are generated. Output signals of the multipliers 141 through 14n are added together by an adding part 20 and the addition result is output.
With reference to FIG. 2, a weight control algorithm is described next. FIG. 2 shows a beam pattern at a base station in a case where a mobile station #2 acting as interference with respect to a desired mobile station #1, and null forming is carried out such that a null point may be directed toward the mobile station #2. In the figure, PTi denotes a transmission power from a mobile station i, PG(θi) denotes a beam gain in an arrival direction θi, and PATT(ri) denotes a distance attenuation amount for a distance ri. SIR1(PT1, PT2, θ1, θ2, r1, r2) denotes an SIR (signal power to interference power ratio) after beam forming in the base station for the mobile station #1 which is a desired one, and is expressed by the following formula (1):SIR1=[PT1−PATT(r1)+PG(θ1)]/[PT2−PATT(r2)+PG(θ2)]  (1)
At this time, assuming that a moving speed vector of the mobile station #2 is fixed, angle velocities, i.e., phase changes Δθ2a and Δθ2b for respective distances rsa and r2b between the base station and mobile station #2 are different from one another, as shown in FIG. 3 (Δθ2a<Δθ2b).
Although FIG. 3 shows a circumferential direction in which the phase change becomes maximum for the purpose of simplification, it is not necessary to limit thereto. Further, the moving speed vector {right arrow over (V)} may have an arbitrary value, and also, there is no limitation to a position of the mobile station. Although the moving speed is assumed to be fixed for the purpose of comparative explanation, generality is maintained even if the moving speed changes at respective positions. Furthermore, although description is made assuming that only the mobile station acting as interference moves for the purpose of simplification of description, the same discussion can be applied, by considering a relative speed of the mobile station #2 with respect to the mobile station #1 even in a case where rather the mobile station #1 which is a desired one moves. Further, in this example, transmission power control (TPC) is carried out so that received power may be fixed in the base station [Pt2−PATT(r2)=Pt1−PATT(r1)].
Japanese Laid-open Patent Application No. 2001-251233 discloses a use of an arrival direction DoA required in receiving, for transmission beam forming in an FDD system using different frequencies for uplink and downlink channels.
Japanese Laid-open Patent Application No. 2001-203630 discloses forming a beam with DoA information, estimating of a position of a mobile station, estimating a traffic therewith as well as DoA information, further forming a beam in a direction in which the traffic is large, and thereby, reducing call collision probability at a time of random access.
Japanese Laid-open Patent Application No. 8-285934 discloses detecting an interference station by collecting information from all the directions at a time of intermittent reception, and carrying out null forming. Japanese Laid-open Patent Application No. 2000-505254 and Japanese Laid-open Patent Application No. 2002-523969 disclose null forming.
Japanese Laid-open Patent Application No. 2003-92548, Japanese Laid-open Patent Application No. 2003-87189 and Japanese Laid-open Patent Application No. 2003-92549 disclose methods of calibration for an adaptive array antenna.
Japanese Laid-open Patent Application No. 2002-508889 discloses beam forming between an own station which receives influence from a new terminal and a terminal with which communication has been already made, when the new terminal for which communication is made newly occurs.
Japanese Laid-open Patent Application No. 2003-51775 discloses forming a null for an interference station, and carrying out steering the thus-formed beam according to a least mean square method.
Japanese Laid-open Patent Application No. 2002-359588 discloses calculating an initial value of beam forming for a terminal for which communication is newly carried out, with the use of beam information for a terminal with which communication has been already carried out, and improving a beam initial pull-in speed.